The vapour density of completely dissociated ${\mathrm{NH}}_4\mathrm{Cl}$ would be

$\frac{\mathrm{Normal} \mathrm{molecular} \mathrm{weight}}{\mathrm{experimental} \mathrm{molecular} \mathrm{weight} }= 1 + \mathrm{\alpha }$

${\mathrm{NH}}_4\mathrm{Cl} \rightleftharpoons {\mathrm{NH}}_3 + \mathrm{HCl}$

$\because \mathrm{\alpha }=1$

$\therefore$    Experimental Molecular wt. = $\frac{\mathrm{nor}. \mathrm{mol}. \mathrm{wt}.}{2}$

Molecular weight $\propto$ Vapour density